Plasma producing device

ABSTRACT

A plasma source includes a plasma source cavity having a tubular shape for forming plasma, an exciting coil for forming a radio frequency magnetic field, and a magnetic path structure for guiding magnetic flux of the radio frequency magnetic field. The magnetic flux extends from an end surface of the plasma source cavity to a side surface of the plasma source cavity and from the side surface of the plasma source cavity to the end surface of the plasma source cavity.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

[0001] The present invention relates to a plasma source, and more specifically, to a radio frequency wave inductively coupled plasma source.

[0002] Many of electronic device fabrication methods utilize plasma to provide various processes such as etching and sputter deposition which are applied to a substrate wafer of semiconductor, glass and other materials. An inductively coupled plasma source is known as one of those plasma activation methods. In most cases of such a plasma source, an excitation coil is located adjacent to a plasma cavity and driven by radio frequency generator to supply electromagnetic power into the cavity. The radio frequency electromagnetic energy will be inductively delivered to ionize source gas and keep activating the plasma in the cavity.

[0003] FIGS. 6(a) and 6(b) describe typical examples of conventional inductively coupled plasma sources. The one shown in FIG. 6(a) uses a solenoid coil 100 as the excitation coil, whereas the other shown in FIG. 6(b) uses a flat coil 104. In the plasma source shown in FIG. 6(a), the solenoid coil 100 is wound around a tubular chamber 103 which works as a plasma source cavity. The solenoid coil 100 is driven by a radio frequency generator 102 through an impedance matching device 101, so that an alternate current induces a magnetic field along the solenoid coil 100 axis in the tubular chamber 103. As a result, plasma P is activated in the tubular chamber 103.

[0004] In the plasma source shown in FIG. 6(b), a flat coil 104 is mounted closely outside the feed through port 106 which is located at the top of the chamber. The radio frequency magnetic field induced by the flat coil 104 penetrates into the chamber 105 through the feed through port 106, so that plasma P is activated inside the chamber 105.

[0005] Recently, the wafer size of the electronic device industries is getting larger. The larger wafer size results in demands for a larger plasma effective area which needs to cover the whole surface of the work wafer in the plasma process equipment as described above. From this point of view, in the case of a plasma source with a solenoid coil 100 shown in FIG. 6(a), it is necessary to increase the diameter of the coil as well as the number of turns, corresponding to a large diameter size, to induce a sufficient magnetic flux density.

[0006] The larger coil has disadvantages in energy loss and impedance matching stability. The energy loss may increase as the coil wire length becomes longer because of the larger resistive element. The impedance matching condition may become critical because the larger reactance and the larger stray capacity of the larger coil size require smaller matching capacitance of the impedance matching network 101 at the fixed frequency, which may reduce the head room for the stable plasma operation condition.

[0007] In the case of a plasma source with a flat coil 104 shown in FIG. 6(b), the induced magnetic flux from a larger coil in both its size and the number of turns converges around the central part of the spiral, which results in a poorer uniformity of the plasma intensity as well as unsuccessful enlargement of the plasma effective area. In addition, it is hard to improve the energy transfer efficiency because the positioning of the plasma source cavity and the flat coil is single sided with each other.

[0008] In view of the above problems, the present invention has been made, and an object of the invention is to provide a radio frequency wave inductively coupled plasma source, wherein the plasma area can be increased without reducing the energy efficiency and uniform plasma can be formed.

[0009] Further objects and advantages of the invention will be apparent from the following description of the invention.

SUMMARY OF THE INVENTION

[0010] In order to attain the objects described above, according to an embodiment of the present invention, a radio frequency wave inductively coupled plasma source includes a plasma source cavity having a tubular shape for forming plasma; an exciting coil for forming a radio frequency magnetic field; and a magnetic path structure for guiding magnetic flux of the radio frequency magnetic field from an end surface of the plasma source cavity to a side surface of the plasma source cavity and from the side surface of the plasma source cavity to the end surface of the plasma source cavity.

[0011] In the radio frequency wave inductively coupled plasma source, the magnetic path structure includes a first soft magnetic material member placed inside the exciting coil for introducing the magnetic flux of the radio frequency magnetic field into the plasma source cavity through the end surface of the plasma source cavity; a second soft magnetic material member placed outside the tubular plasma source cavity for introducing the magnetic flux guided into the plasma source cavity to the side surface of the plasma source cavity; and a third soft magnetic material member placed on a side opposite to the end surface of the plasma source cavity to sandwich the first soft magnetic material member therebetween:for returning the magnetic flux guided to the side surface of the plasma source cavity to the first soft magnetic material member through the second soft magnetic material member.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram showing a plasma source according to an embodiment of the present invention;

[0013] FIGS. 2(a) and 2(b) are views showing a magnetic path structure, wherein FIG. 2(a) shows a first example, and FIG. 2(b) shows a second example;

[0014] FIGS. 3(a) to 3(c) are views showing a radio frequency magnetic field in the plasma source according to the present invention as compared with that in a conventional device, wherein FIG. 3(a) shows a radio frequency magnetic field of the plasma source according to the embodiment of the invention; FIG. 3(b) shows a radio frequency magnetic field of a conventional flat surface type coil; and FIG. 3(c) shows plasma density distributions;

[0015]FIG. 4 is a block diagram showing a plasma source;

[0016]FIG. 5 shows a distribution profile of ion beams obtained in the plasma source shown in FIG. 4; and

[0017] FIGS. 6(a) and 6(b) show conventional plasma sources, wherein FIG. 6(a) shows a plasma source using a solenoid coil, and FIG. 6(b) shows a plasma source using a flat coil.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0018] Hereunder, embodiments of the present invention will be explained with reference to the accompanying drawings. FIG. 1 is a block diagram showing a plasma source according to an embodiment of the present invention, wherein a portion above a line B constitutes a plasma source 1. A plasma source cavity main portion 3 forming a plasma source cavity 2 of a plasma source 1 is placed at an upper portion of a vacuum chamber 12, i.e. a process chamber. The plasma source cavity main portion 3 is made of a ceramic material and the like as used for a feed through port of radio frequency wave power of a conventional device. A radio frequency magnetic field enters the plasma source cavity 2 through the plasma source cavity main portion 3. A vacuum chamber 12 is pumped by a vacuum pump (not shown) connected to an exhaust port 12 a.

[0019] In the plasma source 1 as shown in FIG. 1, the plasma source cavity main portion 3 has a cylindrical shape, and an exciting coil 6 is provided at an outside of the plasma source cavity main portion 3 to face a cylindrical end surface thereof. The exciting coil 6 is a two-turn solenoid type coil, and is connected to an RF power source 9 through an impedance matching device 8. In the present embodiment, the exciting coil 6 is the solenoid type coil. Alternatively, the exciting coil 6 may be, for example, a single turn flat surface type coil. The RF power source 9 uses a frequency of 1 MHz to 100 MHz in view of an economical efficiency. In the present embodiment, a radio frequency power source having a frequency of 13.56 MHz is used.

[0020] A capacitor for matching impedance is provided in the impedance matching device 8, and a capacitance of the capacitor is adjusted to optimize a matching condition. Argon gas is injected into the plasma source cavity 2 to produce plasma. A magnetic path structure 7 is provided around the plasma source cavity main portion 3 for covering the same. The magnetic path structure 7 constitutes a return circuit for magnetic flux generated by the exciting coil 6 (described later), and is made of a soft magnetic material such as iron, nickel, cobalt and the like having good radio frequency properties, i.e. lower loss within an excitation frequency range.

[0021] A grid type aperture electrode 4 is placed between the plasma source cavity 2 and the vacuum chamber 12. A voltage Vacc is applied to the grid 4 from the power source 5. The grid 4 confines the plasma in the plasma source cavity 2, and the acceleration voltage Vacc accelerates ions flying out of the plasma. As a result, the plasma source 1 produces ion beams to be used for various processes. In the present embodiment, a scanning type Faraday cup 10 is provided in the vacuum chamber 12 to measure a current density of the ion beams. An electric current of the ions is measured by a micro-ammeter 11.

[0022] FIGS. 2(a) and 2(b) are views showing the magnetic path structure 7. As shown in FIG. 2(a), the magnetic path structure 7 is formed of three functional portions. The first functional portion is a core portion 7A disposed inside an exciting coil 6 for concentrating magnetic flux at a center portion of the coil and for homogenizing a distribution thereof. Magnetic flux 20 enters the plasma source cavity 2 from a lower end surface of the core portion 7A. The second functional portion is side return portions 7B covering both of the exciting coil 6 and the plasma source cavity main portion 3 for guiding the magnetic flux 20 from an end surface of the core portion 7A to side surfaces of the plasma source cavity main portion 3. The magnetic flux 20 passes through the side surfaces of the plasma source cavity main portion 3, and enters cylindrical side return portions 7B. The third functional portion is a back return portion 7C for allowing the magnetic flux 20 entered the side return portion 7B to return to the core portion 7A.

[0023] The radio frequency energy is emitted from the exciting coil 6 as inductive magnetic field energy, and is supplied to the plasma in the plasma source cavity 2. Incidentally, in FIG. 2(a), the magnetic path structure 7 is shown as a member divided into 7A, 7B and 7C for the respective functions. However, it is not necessarily to divide the magnetic path structure 7 into three portions, and the magnetic path structure 7 may be formed in single piece.

[0024] As shown in FIG. 2(a), upper end surfaces of the side return portions 7B are closely attached to a lower end surface of the back return portion 7C. Alternatively, as shown in FIG. 2(b), the side return portions 7B may be separated from the back return portion 7C. The magnetic path structure 7 made of a soft magnetic material has a magnetic permeability considerably larger than that of air. Accordingly, almost all the magnetic flux 20 getting out of the upper end surfaces of the side return portions 7B enters the back return portion 7C.

[0025] FIGS. 3(a) to 3(c) are views showing a comparison of states of the radio frequency magnetic field between the plasma source 1 according to the present embodiment and a conventional device using a flat coil. FIG. 3(a) corresponds to FIG. 2(a). As described above, the plasma source 1 is provided with the side return portions 7B, so that the magnetic flux 20 getting out of the lower end surface of the core portion 7A is guided to the side return portions 7B. Therefore, it is possible to form the magnetic field having a uniform intensity in the whole plasma source cavity 2. On the other hand, FIG. 3(b) shows a state of the magnetic flux in the conventional device having the flat coil 21. The coil 21 forms the radio frequency magnetic field as shown by the magnetic flux 22 widely spread not only in the plasma source cavity 23 but also in the outer space thereof.

[0026] As described above, in the embodiment of the invention as shown in FIG. 3(a), almost all the magnetic flux 20 getting out of the lower end surface of the core portion 7A enters the side return portions 7B, and enters the opposite end surface of the core portion 7A through the side return portions 7B and the back return portion 7C. In other words, the magnetic flux 20 is present in the magnetic path structure 7 except for the plasma space, and the magnetic field is not formed in an outer space of the device as in the conventional device shown in FIG. 3(b). As a result, the energy efficiency transferred to the plasma is improved.

[0027] Further, the side return portions 7B guide the magnetic flux 20 to the plasma source cavity side surfaces to make the flux density in the plasma source cavity 2 uniform. As a result, the uniform plasma can be formed over a wide area in the plasma source cavity 2. FIG. 3(c) shows a distribution of the plasma density, wherein the horizontal axis represents a position in the plasma source cavity in a radial direction. A curve L1 represents the uniform distribution in the device according to the embodiment of the invention, while a curve L2 represents the widely spread distribution of the conventional device using the flat surface type coil as shown in FIG. 3(b). As shown in FIG. 3(c), the plasma source of the present invention can obtain the uniform distribution of the flux density as compared with the conventional device.

[0028] According to the embodiment of the invention, the plasma source is provided with the magnetic path structure 7, so that the magnetic flux 20 of the radio frequency field formed by the exciting coil 6 is guided to the side return portions 7B of the magnetic path structure 7 through the plasma space. Accordingly, even if the exciting coil 6 does not have a diameter corresponding to a size of a required plasma area, it is possible to allow the magnetic flux 20 to cross the whole plasma area. It is not necessary to increase the diameter of the exciting coil 6. Therefore, it is possible to prevent reactance change and resistance loss due to an increased length of the coil wiring, thereby obtaining stable impedance matching.

[0029] As described above, according to the present embodiment of the present invention, the plasma source 1 is provided with the magnetic path structure 7 to increase the magnetic field intensity induced in the plasma area. In other words, the inductive coupling efficiency of the radio frequency energy with respect to the plasma is increased, and the plasma intensity per unit electric power is increased.

[0030] In the conventional plasma source, the flat coil forms the inductive magnetic field symmetrical relative to the coil surface. The plasma source 1 of the invention is provided with the magnetic path structure 7, so that the asymmetric magnetic field distribution is obtained as shown in FIG. 3 (a). If the plasma source 1 is provided with only the coil 6, it is not possible to obtain such a distribution. In other words, the efficiency of the radio frequency energy is improved, and unnecessary radiation and heating are reduced.

[0031] When the coil 6 is the solenoid coil and does not surround the plasma area, it is still possible to form the induced magnetic field over the whole plasma area due to the magnetic path structure 7. In other words, the magnetic path structure 7 controls the distribution of the inductive magnetic field intensity (plasma intensity distribution) to thereby easily control the uniformity of the plasma activity distribution.

[0032]FIG. 4 shows a specific example of the plasma source 1, wherein dimensions of the respective components of the plasma source are shown. The dimensions of the respective components are as follows: an inner diameter of the plasma source cavity main portion 3 is 220 mm; a diameter of the exciting coil is 200 mm; an outer diameter of the core portion 7A is 180 mm; a diameter of a grid opening portion of the aperture 4 is 210 mm; and a distance between the aperture 4 and the Faraday cup 10 is 300 mm.

[0033] The soft magnetic material constituting the magnetic path structure 7 has relative permeability of 100; the exciting coil 6 has overall self-inductance of 2.0 pH; the radio frequency power source 11 has a frequency of 13.56 MHz; the input radio frequency electric power is 500 W; the accelerating voltage Vacc of the power source 5 is 1.0 kV; and a flow rate of the argon gas is 50 sccm.

[0034] In the structure described above, the Faraday cup 10 was scanned in the radial direction (horizontal direction in the drawing) to measure a distribution profile of an ion beam current density taken out from the plasma source cavity 2. FIG. 5 shows a distribution profile L3 obtained from the measurement. In FIG. 5, the vertical axis represents the ion beam current density having an arbitrary unit (A.U.). The horizontal axis represents a position in the exciting coil 6 in the radial direction with the origin as the center of the exciting coil 6 (center of the plasma source cavity 2).

[0035] When a peak of the distribution profile L3 is assumed to have an intensity of 100%, an area having a relative intensity of more than 90% has a diameter of 170 mm. An area having a relative intensity of 50%, i.e. a half maximum full-width, has a diameter of 190 mm. In other words, it is possible to obtain homogeneous ion beams having a diameter substantially the same as that of the core portion 7A. When the plasma source is provided in an ion beam etching device, the ion beams are uniformly irradiated over the entire area of a base plate having a diameter of about 170 mm, so that it is possible to perform the etching process at a higher speed.

[0036] In the embodiment, the plasma source cavity main portion 3 constitutes a plasma source cavity; the core portion 7A constitutes a first soft magnetic material member; the side return portions 7B constitute a second soft magnetic material member; and the back return portion 7C constitutes a third soft magnetic material member.

[0037] As described above, according to the present invention, the plasma source cavity is provided with the magnetic path structure to obtain the uniform distribution of the magnetic flux in a wide area, so that the plasma is formed in a wide area. Also, the magnetic flux is intensively distributed in the plasma source cavity. without spreading into an unnecessary space outside the plasma source cavity. Therefore, the intensity of the magnetic field induced into the plasma source cavity is increased. Thus, the inductive coupling efficiency of the radio frequency energy is improved with respect to the plasma.

[0038] While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims. 

What is claimed is:
 1. A plasma source, comprising: a plasma source cavity with a tubular shape for producing plasma having an end surface and a side surface, an exciting coil placed adjacent to the plasma source cavity for forming a radio frequency magnetic field, and a magnetic path structure placed outside the plasma source cavity for forming a path of magnetic flux of the radio frequency magnetic field extending from the end surface to the side surface and returning from the side surface to the end surface to thereby provide a uniform distribution of the radio frequency magnetic field in the plasma source cavity.
 2. A plasma source according to claim 1, wherein said magnetic path structure includes a first soft magnetic material member placed inside the exciting coil adjacent to the end surface of the plasma source cavity for guiding the magnetic flux into an inside of the plasma source cavity through the end surface thereof; a second soft magnetic material member disposed adjacent to the side surface of the plasma source cavity for guiding the magnetic flux from the inside of the plasma source cavity to the second soft magnetic material member through the side surface of the plasma source cavity; and a third soft magnetic material member disposed at a side opposite to the end surface of the plasma source cavity with the first soft magnetic material member interposed therebetween for guiding the magnetic flux to the first soft magnetic material member from the second soft magnetic material member.
 3. A plasma source according to claim 2, wherein said first soft magnetic material member projects from the third soft magnetic material member, and the exciting coil is located between the first soft magnetic material member and the second soft magnetic material member.
 4. A plasma source according to claim 3, wherein said first to third soft magnetic material members are formed in single piece. 